Light weight track for a snowmobile

ABSTRACT

A ground engaging track for a vehicle has a continuous loop or endless belt body portion having an exterior surface to engage the ground and an interior surface including driven members. The driven members are configured to be engaged by a drive sprocket to rotate the body portion to move the vehicle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/009,016, filed on Jan. 16, 2008, now U.S. Pat. No. 7,866,766, whichclaims the benefit of U.S. Provisional Application Ser. No. 60/885,165filed on Jan. 16, 2007, both of which are expressly incorporated byreference herein. PCT Application Serial No. PCT/US2007/004607, filed onFeb. 21, 2007, is also incorporated by reference herein.

BACKGROUND AND SUMMARY

The present invention relates to a ground engaging track which is drivenby a drive sprocket to move a vehicle across the ground. Moreparticularly, the present invention relates to a light weight track foruse with an off road vehicle such as, for example, a snowmobile.

Snowmobiles are commonly used for trail riding and utility applications.Trail riding on a snowmobile allows a snowmobile enthusiast to travelthrough areas which are not accessible by other types of vehicles. Forexample, snowmobiles can travel very rapidly across frozen lakes duringthe winter in northern climates. Modern snowmobiles, can cover groundvery rapidly and can cover great distances.

Snowmobiles typically include a ground engaging track driven by thesnowmobile engine to propel the machine. The track is supported beneatha vehicle chassis by a rear suspension which provides a comfortable rideand helps absorb the shock of the snowmobile crossing uneven terrain.

It is desirable to reduce the overall weight of the snowmobile toincrease the range that a snowmobile may cover. The present inventionhelps reduce the weight of the track used to propel the snowmobile.

In one illustrated embodiment of the present invention, a groundengaging track for use with a vehicle having a drive sprocket comprisesa continuous loop body portion having an exterior surface to engage theground and an interior surface including driven members configured to beengaged by the drive sprocket to rotate the body portion and move thevehicle. The body portion is formed from a polymeric material having aYoung's modulus greater than 10,000 psi.

In another illustrated embodiment of the present invention, a groundengaging track for use with a vehicle having a drive sprocket comprisesa body portion having an exterior surface to engage the ground and aninterior surface having driven members configured to be engaged by thedrive sprocket to rotate the body portion and move the vehicle. The bodyportion is molded from a polymeric material having a Young's modulusgreater than 10,000 psi. The track further comprises a plurality ofreinforcement members molded into the body portion.

In yet another illustrated embodiment of the present invention, a groundengaging track for use with a vehicle having a drive sprocket comprisesa body portion having an interior surface including a plurality of drivelugs. The plurality of drive lugs are longitudinally spaced apart toprovide driven members configured to be engaged by the drive sprocket torotate the body portion and move the vehicle. The body portion and theplurality of drive lugs are integrally formed from a polymeric materialhaving a degree of crystallinity greater than or equal to 50%.

In still another illustrated embodiment of the present invention, aground engaging track for use with a vehicle having a drive sprocketcomprises an endless belt body portion having an interior surfaceincluding a plurality of drive lugs formed integrally with the bodyportion. The plurality of drive lugs are longitudinally spaced apart toprovide driven members configured to be engaged by the drive sprocket torotate the body portion and move the vehicle. The body portion also hasan exterior surface including a plurality of tread lugs formedintegrally with the body portion and configured to engage the ground. Afirst set of tread lugs is longitudinally aligned with the drive lugsand a second set of tread lugs being longitudinally aligned with spacesbetween adjacent drive lugs.

In a further illustrated embodiment of the present invention, a groundengaging track for use with a vehicle having a drive sprocket comprisesan endless belt body portion having an exterior surface to engage theground and an interior surface having driven members configured to beengaged by the drive sprocket to rotate the endless belt body portionand move the vehicle. The body portion has a thickness less than 0.2inch.

In a still further illustrated embodiment of the present invention, aground engaging track for use with a vehicle having a drive sprocketcomprises a continuous loop body portion having an exterior surface toengage the ground and an interior surface including members configuredto be engaged by the drive sprocket to rotate the body portion and movethe vehicle. The body portion is formed from a polymeric material havinga molecular weight greater than two million Daltons.

Additional features of the invention will become apparent to thoseskilled in the art upon consideration of the following detaileddescription of the best mode of carrying out the invention as presentlyperceived.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figuresin which:

FIG. 1 is an exploded perspective view of an illustrated embodiment of aground engaging track and a drive sprocket of the present invention foruse with a vehicle;

FIG. 2 is a cross-sectional view of another illustrated embodiment ofthe present invention illustrating another exemplary ground engagingtrack for use with a vehicle having a drive sprocket; and

FIG. 3 is another cross-sectional view further illustrating the groundengaging track and drive sprocket of FIG. 2.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring now to the drawings, FIG. 1 illustrates one embodiment of aground engaging track 10 for use with a vehicle (not shown) having adrive sprocket 12. It should be appreciated that while a single drivesprocket 12 is shown, multiple drive sprockets may be implemented inother embodiments. It is understood that track 10 may be formed inconfigurations other than those described herein.

The vehicle typically includes an engine (not shown), such as aninternal combustion engine, that is operatively connected to the track10 via a drive axle 11. In a snowmobile embodiment, a continuouslyvariable transmission (not shown) illustratively connects the engine tothe drive axle 11 in a conventional manner. Drive sprocket 12 is mountedto the drive axle 11 for common rotational movement about a drivesprocket axis 13.

As illustrated in FIG. 1, drive sprocket 12 includes central hub 15 anda plurality of Y-shaped projections 17. Branches of the Y-shapedprojections define a recessed portion 32 and spaced apart pairs ofsprocket teeth 19, 21 that are configured to engage corresponding pairsof drive members 26 formed on track 10 as discussed below. Sprocketteeth 19, 21 therefore drive the track 10 upon rotation of the sprocket12 about drive sprocket axis 13.

The sprocket 12 has a central bore 23 formed in hub 15 through which thedrive axle 11 extends. The drive axle 11 is rigidly secured to thesprocket 12 in a conventional manner. The sprocket 12 and the Y-shapedprojections 17 defining sprocket teeth 19, 21 are illustrativelyintegrally formed from a strong, light material such as aluminum orplastic. Sprocket 12 may also be formed from other suitable materialsuch as steel or a composite including carbon fibers. In other words,the exact composition of the sprocket 12 may be selected from a widevariety of substances without departing from the scope of the presentinvention. In addition, the sprocket teeth 19, 21 may be formedseparately from the sprocket body and subsequently rigidly fastened (viaglue, rivets, welds, bolts, etc.) to the sprocket 12.

Illustratively, drive sprocket 12 includes a central channel 30 definedby spaced apart walls 34. Channel 30 receives centering lugs 28 of track10 therein as the sprocket 12 rotates as discussed below. The channel 30may be implemented in other suitable configurations, if desired. Teeth19, 21 of drive sprocket 12 are formed to include apertures 42 therein.Drive sprocket 12 defines cavities 44 in communication with apertures 42for removal of snow, dirt and other debris as described in greaterdetail below.

FIG. 1 also illustrates track 10 including body portion 14 having anexterior surface 16 and an interior surface 18. Track 10 isillustratively formed from a polymeric material having a high elasticmodulus as discussed below. Illustratively, the polymeric material has aYoung's modulus greater than 10,000 psi and preferably much higher asdiscussed below. Preferably, the polymeric material is an ultra highmolecular weight polyethylene.

In one embodiment, body portion 14 is molded or otherwise formed as acontinuous loop. In another embodiment, body portion is an endless belttrack. For purposes of this application, the term “continuous loop”defines a monolithic structure without connectors, couplers or splices.For purpose of this application, the term “endless belt” may includeconnectors, couplers or splices. In any event, it should be appreciatedthat alternative embodiments may be implemented in any other suitableconfiguration.

As previously mentioned, body portion 14 has exterior surface 16 andinterior surface 18. The thickness of the body portion 14 of track 10(distance between exterior surface 16 and interior surface 18) asillustrated by dimension T is less preferably than about 0.2 inch.Illustratively, the thickness is preferably between about 0.2 inch andabout 0.1 inch. As illustrated in FIG. 1, body portion 14 is thin enoughto be flexible.

Exterior surface 16 includes a plurality of tread lugs 20 configured toengage the ground. Tread lugs 20 are illustratively integrally formedwith body portion 14 of track 10. Therefore, tread lugs 20 are alsoformed from the same polymeric material as the body portion. Asillustrated in FIG. 1, tread lugs 20 protrude from the exterior surface16 of body portion 14. It should also be appreciated that tread lugs 20may have different shapes and sizes than illustrated in FIG. 1.

Optionally, tread tips 22 are coupled to tread lugs 20. Tread tips 22are illustratively formed from a second material which is more elasticthan the polymeric material of body portion 14 and tread lugs 20.Illustratively, the elastomeric second material has a Young's modulusless than or equal to 5,000 psi and preferably less than or equal to2,000 psi. For example, tips 22 may be formed from rubber having aYoung's modulus of about 1,500 psi.

Tread tips 22 are either co-molded to tread lugs 20 or fastened to treadlugs 20 by any suitable means, such as adhesive. The optional tread tips22 provide greater traction on hard or icy surfaces. The optional treadtips 22 also reduce noise when track 10 is operated over hard surfacescompared to an embodiment of the track 10 without the tips 22 on thetread lugs 20.

Interior surface 18 includes integrally formed drive lugs 24 whichprovide driven members 26 that are configured to engage drive sprocketteeth 19, 21. Centering lugs 28 are located between the drive members26. Drive lugs 24 and centering lugs 28 are illustratively integrallyformed from the same polymeric material as the body portion 14. Drivemembers 26 are configured to be engaged by drive sprocket teeth 19, 21as the sprocket 12 rotates to rotate body portion 14 and move thevehicle. Centering lugs 28 are configured to enter the central channel30 of drive sprocket 12 to maintain position of body portion 14 of track10 relative to drive sprocket 12.

Optionally, centering tips 29 are coupled to centering lugs 28.Centering tips 29 are illustratively formed from a second material whichis more elastic than the polymeric material of body portion 14, drivelugs 24 and centering lugs 28. Illustratively, the elastomeric secondmaterial has a Young's modulus less than 5,000 psi and preferably lessthan 2,000 psi. For example, tips 29 may be formed from rubber having aYoung's modulus of about 1,500 psi.

Centering tips 29 are either co-molded to centering lugs 28 or fastenedto centering lugs 28 by any suitable means, such as adhesive. Theoptional centering tips 29 may provide greater traction against drivesprocket 12 and may be quieter than an embodiment of track 10 includingcentering lugs 28 without the tips.

In another embodiment, a track (not shown) is formed to include driveapertures adjacent drive lugs 24. This track embodiment is driven by asprocket as illustrated in U.S. Pat. No. 7,051,829 to Wahl which isincorporated herein by reference. In this embodiment, centering lugs 28are not typically used.

As shown in FIG. 1, drive members 26 are spaced apart longitudinallyalong track 10 disposed along lateral axes 40. Tread lugs 20 aresimilarly spaced apart along track 10 disposed along lateral axes 40. Itshould be appreciated that while tread lugs 20 are shown disposed alonglateral axes 40, tread lugs 20 may be implemented at a different,illustratively, higher frequency, as discussed in more detail in FIG. 3.Track 10 is capable of supporting tread lugs 20 at a higher frequency,in part because of flexibility provided by the polymeric materialcomposition of track 10.

A plurality of tread lugs 20 are illustratively disposed along a singlelateral axis 40. A plurality of sets of tread lugs 20 are disposedparallel to other sets of tread lugs 20. It should be appreciated thattread lugs 20 may be implemented in other suitable configurations.

Optionally, reinforcement members 38 are illustratively embedded withinbody portion 14. Reinforcement members 38 are illustratively formed frompolymeric material having a Young's modulus greater than about 75,000psi and preferably greater than about 95,000 psi. For example,reinforcement members 38 are made from aramid fibers or a polyestermaterial.

During operation, snow, dirt and other debris can collect on theinterior surface 18 of track 10, on drive members 26, and on drivesprocket 12. Apertures 42 formed in sprocket teeth 19, 21 are incommunication with cavities 44 to allow snow, dirt and other debris tomove through apertures 42 in the directions of arrows 46 as the sprocket12 rotates to move the track 10. Movement of the snow, dirt or otherdebris in the direction of arrows 46 allows for removal of the snow,dirt and other debris from interior surface 18, drive members 26 anddrive sprocket 12.

FIGS. 2 and 3 illustrate another embodiment of a ground engaging track110 for use with a vehicle (not shown) having a drive sprocket 112. Thisembodiment is substantially similar to the previous embodiment andprovides additional features applicable to all embodiments. It isunderstood that this embodiment may incorporate all of the features ofthe previous embodiment.

FIG. 2 illustrates track 110 including body portion 114 having exteriorsurface 116 and interior surface 118. Exterior surface 116 includestread lugs 120 configured to engage the ground. Tread lugs 120 define agenerally convex ground engaging surface as illustrated by dashed line148. Convex ground engaging surface illustrated by dashed line 148allows for pivoting of track 10 against ground G. Optionally tread tips122 are coupled to tread lugs 120.

Still referring to FIG. 2, interior surface 118 includes integrallyformed drive lugs 124 including drive members 126 configured to beengaged by teeth 125 of drive sprocket 112 as the sprocket 112 rotates.Only half of the drive sprocket 112 is shown in FIG. 2 with the otherhalf being a mirror image. Drive lugs 124 provide driven members 126.Centering lugs 128 are also integrally formed with interior surface 118of track 110. Centering lugs 128 are configured to enter a centralchannel 134 of drive sprocket 112 to maintain position of body portion114 relative to drive sprocket 112. Optional centering tips 129 arecoupled to centering lugs 128 as discussed above. The optional centeringtips 129 provide quieter operation compared to an embodiment of track110 including centering lugs 128 alone without the optional centeringtips 129.

Illustratively, drive sprocket 112 is coupled to drive axle 150.Sprocket includes a plurality of teeth 125 which are configured toengage the drive members 126 as the sprocket 112 rotates. Teeth 125include lateral members 136 having a ramp surface 138 which isconfigured to aid in removal of snow, dirt and other debris. Duringoperation, snow, dirt and other debris can deposit on the interiorsurface 118, on drive members 124, and on drive sprocket 112. Spacebetween lateral member 136 and interior surface 118 and the ramp surface138 facilitate movement of snow, dirt and debris in the direction ofarrow 146 to remove the snow, dirt and other debris from interiorsurface 118, drive members 124 and drive sprocket 112.

Still referring to FIG. 2, body portion 114 optionally includes embeddedreinforcement members 38 to increase the tensile strength of bodyportion 114 and minimize longitudinal stretching of track 110. WhileFIG. 2 shows reinforcement members 38 arranged longitudinally, it shouldbe understood that the arrangement of reinforcement members 38 inrelation to body portion 114 is not limited to any arrangement.Reinforcement members 38 are optionally made from aramid fibers orpolyester.

FIG. 3 shows a cross sectional view of the ground engaging track 110 ofFIG. 2. As previously disclosed, track 110 includes body portion 114which includes, among other things, drive members 126. Drive sprocket112 includes teeth 125 which are configured to engage drive members 126as drive sprocket 112 rotates to rotate body portion 114 and move thevehicle (not shown).

As also shown in FIG. 3, track 110 includes tread lugs 120. It should beunderstood from reference to FIGS. 1, 2 that FIG. 3 illustrates severalrows of tread lugs 120. It should also be noted that there are more rowsof tread lugs 120 than drive members 126. First sets 160 of tread lugs120 are longitudinally aligned with the drive lugs 124 and second sets162 of tread lugs 120 are longitudinally aligned with spaces 164 betweenadjacent drive lugs 124. Tread lugs 120 formed on exterior surface 116may be located between drive members 126 due to the flexibility providedby the polymeric material composition of track 110.

DESCRIPTION OF THE POLYMERIC MATERIAL OF THE TRACK

As discussed above, the body portion 14, 114 of the track and well astread lugs 20, 120, drive lugs 24, 124 and centering lugs 28, 128 areillustratively integrally formed from a polymeric material having aYoung's modulus greater than 10,000 psi. In other illustratedembodiments the polymeric material has a Young's modulus greater than orequal to 25,000 psi, greater than or equal to 50,000 psi, or greaterthan or equal to 75,000 psi. In preferred embodiments, the polymericmaterial is ultra high molecular weight polyethylene having a Young'smodulus greater than about 95,000 psi.

The polymeric material of the body portion 14, 114, tread lugs 20, 120,drive lugs 24, 124 and centering lugs 28, 128 may also be defined interms of the degree of crystallinity of the polymeric material.Illustratively, the polymeric material has a degree of crystallinitygreater than or equal to 50%. In other illustrated embodiments thepolymeric material has a degree of crystallinity between about 55% and85%. In preferred embodiments, the polymeric material has a degree ofcrystallinity greater than or equal to 85%

The polymeric material of the body portion 14, 114, tread lugs 20, 120,drive lugs 24, 124 and centering lugs 28, 128 may also be defined interms of the molecular weight of the polymeric material. The polymericmaterial illustratively has a molecular weight greater than two millionDaltons. In other illustrated embodiments the body portion is formedfrom a polymeric material having a molecular weight greater than threemillion Daltons, greater than four million Daltons, or greater than fivemillion Daltons. In preferred embodiments, the polymeric material isultra high molecular weight polyethylene having a having a molecularweight between about three million Daltons and about 5.7 millionDaltons.

The properties of the polymeric material discussed herein permit thetrack to be formed from a thin sheet of the polymeric material.Illustratively, the body portion having a thickness less than 0.2 inch.Preferably, the body portion has a thickness between about 0.2 inch andabout 0.1 inch.

It is understood that a track made of the polymeric material describedherein is not limited to the specific track configurations illustratedherein. Any other configurations of drive lugs, tread lugs, andcentering lugs may be used in accordance with the present invention. Asdiscussed above, any desired sprocket configuration may also be usedincluding multiple drive sprockets as is well known in the art. It isalso understood that the track may be formed with or without centeringlugs, depending on the track design.

Although the invention has been described in detail with reference tocertain illustrated embodiments, variations and modifications existwithin the scope and spirit of the present invention as described anddefined in the following claims.

1. A ground engaging track for use with a vehicle having a drivesprocket, the track comprising a continuous loop body portion having anexterior surface to engage the ground and an interior surface includingdriven members configured to be engaged by the drive sprocket to rotatethe body portion and move the vehicle, the body portion being formedfrom a polymeric material having a Young's modulus greater than 10,000psi.
 2. The track of claim 1, wherein the body portion is formed from apolymeric material having a Young's modulus greater than or equal to25,000 psi.
 3. The track of claim 1, wherein the body portion is formedfrom a polymeric material having a Young's modulus greater than or equalto 50,000 psi.
 4. The track of claim 1, wherein the body portion isformed from a polymeric material having a Young's modulus greater thanor equal to 75,000 psi.
 5. The track of claim 1, wherein the bodyportion is formed from a polymeric material having a Young's modulusgreater than or equal to 95,000 psi.
 6. The track of claim 1, whereinthe body portion is formed from a polymeric material having a Young'smodulus greater than or equal to 100,000 psi.
 7. The track of claim 1,wherein the body portion is formed from a polymeric material having aYoung's modulus greater than or equal to 110,000 psi.
 8. The track ofclaim 1, wherein the body portion is formed from a polymeric materialhaving a Young's modulus greater than or equal to 125,000 psi.
 9. Thetrack of claim 1, wherein the interior surface of the body portionincludes a plurality of drive lugs which provide the driven members, theplurality of drive lugs also being formed from the same polymericmaterial.
 10. The track of claim 9, further comprising a second materiallocated on at least one of the plurality of drive lugs, the secondmaterial being more elastic than the polymeric material.
 11. The trackof claim 1, wherein the exterior surface of the body portion includes aplurality of tread lugs configured to engage the ground, the pluralityof tread lugs also being formed from the same polymeric material as thebody portion.
 12. The track of claim 11, further comprising a secondmaterial located on at least one of the plurality of tread lugs, thesecond material being more elastic than the polymeric material.
 13. Thetrack of claim 12, wherein the second material is molded onto theexterior surface of the body portion.
 14. The track of claim 12, whereinthe second material has a Young's modulus less than or equal to 5,000psi.
 15. The track of claim 12, wherein the second material has aYoung's modulus less than or equal to 2,000 psi.
 16. A ground engagingtrack for use with a vehicle having a drive sprocket, the trackcomprising: a body portion having an exterior surface to engage theground and an interior surface having driven members configured to beengaged by the drive sprocket to rotate the body portion and move thevehicle, the body portion being molded from a polymeric material havinga Young's modulus greater than 10,000 psi; and a plurality ofreinforcement members molded into the body portion.
 17. The track ofclaim 16, wherein the plurality of reinforcement members include aplurality of first elongated members extending in a first direction anda plurality of second elongated members extending in a second directiongenerally orthogonal to the first direction.
 18. The track of claim 16,wherein the plurality of reinforcement members are formed from an aramidfiber.
 19. The track of claim 16, wherein the plurality of reinforcementmembers are formed from a polyester having a Young's modulus greaterthan 75,000 psi.
 20. The track of claim 16, wherein the interior surfaceis formed to include a plurality of drive lugs formed integrally withthe belt, the plurality of drive lugs being longitudinally spaced apartto provide the driven members, and the exterior surface is formed toinclude a plurality of tread lugs formed integrally with the belt andconfigured to engage the ground, a first set of tread lugs beinglongitudinally aligned with the drive lugs and a second set of treadlugs being longitudinally aligned with spaces between adjacent drivelugs.
 21. A ground engaging track for use with a vehicle having a drivesprocket, the track comprising a body portion having an interior surfaceincluding a plurality of drive lugs, the plurality of drive lugs beinglongitudinally spaced apart to provide driven members configured to beengaged by the drive sprocket to rotate the body portion and move thevehicle, the body portion and the plurality of drive lugs beingintegrally formed from a polymeric material having a degree ofcrystallinity greater than or equal to 50%.
 22. The track of claim 21,wherein the polymeric material has a degree of crystallinity greaterthan or equal to 55%.
 23. The track of claim 21, wherein the polymericmaterial has a degree of crystallinity greater than or equal to 60%. 24.The track of claim 21, wherein the polymeric material has a degree ofcrystallinity greater than or equal to 62%.
 25. The track of claim 21,wherein the polymeric material has a degree of crystallinity greaterthan or equal to 85%.
 26. A ground engaging track for use with a vehiclehaving a drive sprocket, the track comprising an endless belt bodyportion having an exterior surface to engage the ground and an interiorsurface having driven members configured to be engaged by the drivesprocket to rotate the endless belt body portion and move the vehicle,the body portion having a thickness less than 0.2 inch.
 27. The track ofclaim 26, wherein the belt has a thickness of less than or equal to0.175 inch.
 28. The track of claim 26, wherein the belt has a thicknessof less than or equal to 0.15 inch.
 29. The track of claim 26, whereinthe belt has a thickness of about 0.10 inch.
 30. A ground engaging trackfor use with a vehicle having a drive sprocket, the track comprising acontinuous loop body portion having an exterior surface to engage theground and an interior surface including members configured to beengaged by the drive sprocket to rotate the body portion and move thevehicle, the body portion being formed from a polymeric material havinga molecular weight greater than two million Daltons.
 31. The track ofclaim 30, further comprising a plurality of reinforcement members moldedinto the body portion.
 32. The track of claim 30, wherein the sprocketincludes a central recessed portion and the interior surface of the bodyportion includes a plurality of centering lugs which cooperate with thecentral recessed portion of the sprocket to maintain a position of thebody portion relative to the sprocket, the plurality of centering lugsbeing formed from the same polymeric material as the body portion. 33.The track of claim 32, further comprising a second material located onat least one of the plurality of centering lugs, the second materialbeing more elastic than the polymeric material.
 34. The track of claim33, wherein the second material is molded onto the interior surface ofthe body portion onto at least one of the centering lugs.
 35. The trackof claim 33, wherein the second material is amorphic.
 36. The track ofclaim 33, wherein the second material is rubber.
 37. The track of claim33, wherein the second material has a Young's modulus less than or equalto 5,000 psi.
 38. The track of claim 33, wherein the second material hasa Young's modulus less than or equal to 2,000 psi.
 39. The track ofclaim 30, wherein the body portion is formed from a polymeric materialhaving a molecular weight greater than three million Daltons.
 40. Thetrack of claim 30, wherein the body portion is formed from a polymericmaterial having a molecular weight greater than four million Daltons.41. The track of claim 30, wherein the body portion is formed from apolymeric material having a molecular weight greater than five millionDaltons.
 42. The track of claim 30, wherein the polymeric material isultra high molecular weight polyethylene.